Out of the 76% that tested postive, 52% contained the virus at low levels. It cannot be discovered whether the virus in the oysters is infectious or not and a safe limit for norovirus has not been established.
The FSA warned consumers to be aware of the risks of eating oysters raw, as they are traditionally, but no other advice has been given. The virus is highly infectious and generally spread from person to person, although a proportion of cases are contracted through eating contaminated food.
Controls before and after commercial harvesting of oysters provide good protection against harmful bacteria, but are less effective at removing viruses from live shellfish.
The oysters were collected from 39 harvesting areas across the UK, giving a sample of 8,000.
“This research is the first of its kind in the UK,” said FSA chief scientist Andrew Wadge.
“It will be important to help improve the knowledge of the levels of norovirus found in shellfish at production sites. The results, along with data from other research, will help us work with producers to find ways to reduce the levels of norovirus in shellfish, and work within Europe to establish safe levels.”
A specific legal safe level for norovirus in oysters will be presented to the European Commission.
The virus can build up in oysters as they filter large volumes of water to get their food, collecting bacteria and viruses in the water.